A boosted braking device for a motor vehicle having a master cylinder (200) and a pneumatic booster (100). The master cylinder (200) is equipped with a primary piston (30) which receives an actuating force to increase pressure in a primary hydraulic circuit (I) which increases an increase in pressure in a secondary hydraulic circuit (II) and a secondary piston (31) which is subjected to the hydraulic pressure in the primary circuit (I). The pneumatic booster (100) is controlled by the application of an input force through which an actuation force is applied to the primary piston (30). The primary piston (30) has a hollow cylinder (200) the interior of which is in communication with the primary hydraulic circuit (I). A reaction piston (34) which slides in a leaktigth fashion inside of the hollow cylinder (200) urged toward the master cylinder by a first elastic force provided by a first elastic member (46). The hollow cylinder (32) has at least one opening (36) through which the interior thereof is in communication with the inside of the master cylinder (200). The hollow cylinder (32) includes a reaction piece (76) against which an end part (58) of the reaction piston (34) abuts to exert a force proportional to a force caused by the hydraulic pressure in the secondary circuit (II) exerted on the secondary piston (31) in the event of a failure of the primary hydraulic circuit(I).